The subject matter disclosed herein relates generally to tuning a motor drive and, more specifically, to a method for tuning of a high performance motor drive system utilizing frequency response analysis.
As is known to those skilled in the art, motor drives are utilized to control operation of a motor. According to one common configuration, a motor drive includes a DC bus having a DC voltage of suitable magnitude from which an AC voltage may be generated and provided to the motor. The DC voltage may be provided as an input to the motor drive or, alternately, the motor drive may include a rectifier section which converts an AC voltage input to the DC voltage present on the DC bus. The motor drive includes power electronic switching devices, such as insulated gate bipolar transistors (IGBTs), thyristors, or silicon controlled rectifiers (SCRs). The power electronic switching device further includes a reverse conduction power electronic device, such as a free-wheeling diode, connected in parallel across the power electronic switching device. The reverse conduction power electronic device is configured to conduct during time intervals in which the power electronic switching device is not conducting. A controller in the motor drive generates switching signals to selectively turn on or off each switching device to generate a desired DC voltage on the DC bus or a desired motor voltage.
The motor drive receives a command signal which indicates the desired operation of the motor. The command signal may be a desired position, speed, or torque at which the motor is to operate. The position, speed, and torque of the motor are controlled by varying the amplitude and frequency of the AC voltage applied to the stator. The motor is connected to the output terminals of the motor drive, and the controller generates the switching signals to rapidly switch the switching devices on and off at a predetermined switching frequency and, thereby, alternately connects or disconnects the DC bus to the output terminals and, in turn, to the motor. By varying the duration during each switching period for which the output terminal of the motor drive is connected to the DC voltage, the magnitude of the output voltage is varied. The motor controller utilizes modulation techniques such as pulse width modulation (PWM) to control the switching and to synthesize waveforms having desired amplitudes and frequencies.
In order to convert the command signal to the desired output voltage, the motor drive includes a control section. The control section may vary in complexity according to the performance requirements of the motor drive. For instance, a motor drive controlling operation of a pump may only need to start and stop the pump responsive to an on/off command. The motor drive may require minimal control such as an acceleration and deceleration time for the pump. In contrast, another motor drive may control a servo motor moving, for example, one axis of a machining center or an industrial robotic arm. The motor drive may need to not only start and stop the motor, but operate at various operating speeds and/or torques or follow a position command. The motor control may include multiple control circuits, such as a position, velocity, torque, or current control circuit, or a combination thereof. Each control circuit may include, for example, a proportional (P), integral (I), or derivative (D) control path with associated controller gains in each path and may further require additional feedback and/or feed forward control gains. In order to achieve the desired operating performance of the motor, it is necessary to properly select the control paths and the control gains associated with each path.
However, selecting the control gains to achieve a desired level of performance has certain challenges. Although the control paths may be either in parallel or in series with each other, there is ultimately a single input and a single output for the control system. Adjusting a control gain along one path impacts the performance of one or more other paths. The interaction of control gains along various paths often requires a time and labor-intensive iterative approach to selecting control gains in order to achieve the desired level of performance. Further, the final control gains selected often result in less than optimum performance in order to avoid approaching operating regions in which one of the control gains may result in instability of the control system.
In order to achieve better performance from a control system, some controllers provide an automatic tuning procedure. However, existing automatic tuning procedures have certain drawbacks as well. The controller typically only knows the characteristics of a portion of the controlled system. For example, the controlled system may include hardware contained within the motor controller, a motor, a physical coupling to a controlled load, and a controlled load. The controller may know the characteristics of the hardware within the motor controller and of the motor, but may be required to make assumptions regarding the rest of the controlled system. For example, the auto tuning procedure may expect “rigid” coupling of the controlled load to the motor or a low inertia ratio. Such characteristics of the controlled system more closely represent an ideal system and reduce the complexity of determining controller gains. If a load has “compliant” coupling to the motor or has a high inertia ratio, various resonant operating points may exist. When a resonant operating point exists, automatic tuning procedures typically either fail to identify controller gains or identify controller gains that result in a slow response from the controlled load in order to avoid exciting the resonant operating point. This poor tuning results in a system being controlled at less than its desired performance level, reducing the overall performance of the system.
Thus, it would be desirable to provide a system to perform automatic tuning of a motor controller that selects controller gains to achieve a desired level of performance.
Even if controller gains are initially selected to achieve a desired level of performance, the dynamics of the controlled system may vary over time. The dynamics of the system may vary due, for example, to wear of mechanical equipment or repair and/or replacement with equipment having different characteristics than the original equipment. Environmental conditions, such as temperature and/or humidity may also lead to variable operation of the controlled system.
Thus, it would also be desirable to provide a system to perform adaptive tuning of a motor controller to periodically refine the selected controller gains during operation of the controlled system to maintain the desired level of performance.